1. Field of Invention
The present invention relates to a motor control device, and, in particular, relates to a motor control device having an optimal protection function in accordance with an operational state of a motor.
2. Description of the Related Art
If, when operating a motor, an overloaded state continues, the temperature of motor windings may continue to rise, possibly leading to burning out of the motor windings. In consideration of this possibility, thermal protection devices which use a temperature gauge, such as a thermostat or thermistor embedded in the motor winding, to cut off the flow of current to the motor when the temperature rises have come to be used.
Additionally, an electronic thermal system in which no temperature detector is used, such as that disclosed by Japanese Patent Application Laid-Open No. 9-261850 or Japanese Patent Application Laid-Open No. 6-253577, can be used. Such a system is intended to thermally protect heat-generating components having a thermal time constant of several seconds or more and their surroundings, and protects objects to be protected by calculating a heating value and a heat release amount to estimate the temperature based on a history of current commands or the like.
FIG. 5 is a block diagram of an example motor control device to which an electronic thermal system is applied. A motor 44 is controlled based on a speed command V* calculated by a high-level controller (such as position control), not shown in FIG. 5. A flow of control will be described below.
A speed controller 41 receives a speed feedback V calculated by a converter 46 using apposition detected value of an encoder 45 and the speed command V* as input to perform operations such as PI control and calculate a current command I*. Next, a current controller 42 receives a current feedback I and the current command I* to calculate an inverter drive command. Based on the inverter drive command, an inverter circuit 43 outputs power waveforms to drive the motor 44. Also, based on the current command I*, a temperature estimation component 47 estimates the temperature. If the estimated temperature reaches a threshold, the temperature estimation part 47 outputs an operation stop command of the motor 44 to the current controller 42.
In a system using the thermal protection device described above, it is assumed that the temperature detected by a thermistor or a thermostat used as a temperature detector accurately reflects the temperature of the motor winding itself. Thus, if there is a temperature difference between the temperature detector and motor winding, the protection function will not work properly. Also, if motor rotation is frozen as a result, for example, a collision, current may be concentrated on one phase, causing the temperature of a portion of the motor winding to rise. As a result, a temperature difference arises between the portion of the motor winding where the temperature rises and the temperature detector, such that the temperature detector cannot detect the temperature of the motor winding correctly, possibly leading to burning out of the motor winding. Further, if thermal conduction from the motor winding to the temperature detector is poor or the thermal time constant of the motor winding is smaller than the detection delay of the temperature detector, the temperature detector cannot detect the temperature of the motor winding correctly, in which case it may not be possible to prevent burning out, or possible actual combustion, of the motor winding.
In a conventional electronic thermal system described above, because, when the motor is rotating, current flows equally in three phases (U phase, V phase, and W phase) and the temperature of the motor winding rises as a whole, the temperature estimation component 47 can accurately estimate the temperature of the motor winding. A motor protection curve in this case looks like the curve shown in FIG. 4. Here, M4, A4, and T4 in FIG. 4 are values determined by the motor. Here, the value M4 is a motor current limiting value, the value 4 is typically set to the motor continuous rating current, and the value T4 is set depending on the motor. For example, the protection curve may be determined based on an actually measured thermal time constant when the motor is rotating at a continuous rated power. However, if the motor is locked, current concentrates in one phase to raise the temperature of a portion of the motor winding, leading to burning out of the motor winding. Thus, it is necessary to reexamine (lower) the threshold to prevent burning out of the motor winding when the motor is locked. However, if the threshold is lowered and the motor is rotating, the estimated temperature value may exceed the threshold even when the motor is operating normally, causing a problem of unnecessary activation of the protection function.
Moreover, in the electronic thermal system, the estimated value of temperature of the motor winding is the same for all three phases regardless of the operational state of motor. If, at this point, current concentrates in one phase, the temperature of the motor winding deviates from the estimated value of temperature, causing a problem of burning out of the motor winding before activating the protection function.
The present invention was made to solve the above problems, and an object thereof is to provide an optimal motor protection function.